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Energy consumption is of utmost importance in ad hoc wireless networks where battery-operated nodes have limited power resources. Energy consumption is also important in meshed WLAN networks such as emerging home entertainment networks and meshed WiMax metropolitan area networks. So far, it has been advocated that nodes have to operate at the lowest possible power level that keeps the network connected in order to best conserve energy in the system. This approach however results in increased internal traffic and forces more nodes to be awake to relay information to the intended receivers. In this paper, we investigate the effects of the transmit power and the transmit data rate on the bit-per-joule performance of IEEE 802.11 based ad hoc wireless networks. We assume nodes to use software controlled radios whose modulation/coding scheme and rate parameters can be dynamically selected. We then develop an analytical model to calculate the energy consumption rate in the network. Our model takes into account the energy spent when the nodes are idle or asleep, when transmitting control packets, as well as when relaying, retransmitting and receiving packets. We examine the performance efficiency of the throughput per unit energy consumed attained under the employment of different jointly selected transmit power levels and data rates. We then compare the results attained using our analytical model with results attained via simulation using transmit power levels and data rate values found in commercially available IEEE 802.11b WLAN cards. Our results show that, when traffic is uniformly distributed across the area of operations, increasing the nodal transmit power level increases the bits/sec per watt, (or, equivalently, the bit per joule) performance of the network.